1. Field of the Invention
The present invention relates to an optical receiving station and an optical communication system for compensating for chromatic dispersion in an optical transmission line in an optical transmission system where an optical duo-binary modulation method is used. The invention also relates to a dispersion compensating method for adjusting the total dispersion amount of an optical transmission line.
At present, optical communication apparatuses capable of transmitting a large amount of optical signals over an ultra-long distance are required for construction of future multimedia networks. Optical transmission systems of 10 Gb/s have been put in practical use in current trunk line optical communication, to satisfy the above requirement. Further, time division multiplexing optical transmission systems of 40 Gb/s have been studied and developed.
2. Description of the Related Art
The maximum transmission distance of an optical fiber without relaying is limited by attenuation and chromatic dispersion of an optical signal. To increase the transmission distance, it is necessary to compensate for the attenuation and chromatic dispersion. The attenuation is compensated by a rare-earth-element-doped fiber amplifier or the like. On the other hand, the chromatic dispersion is compensated by inserting, in an optical transmission line, a dispersion compensator having a fixed dispersion characteristic of an opposite sign to the sign of the value of dispersion received by an optical signal traveling through an optical fiber.
Incidentally, being in inverse proportion to the square of the bit rate of an optical signal, the dispersion compensation tolerance at 40 Gb/s is as small as 1/16 of that at 10 Gb/s. This makes it necessary to precisely adjust the dispersion value of a dispersion compensator.
The dispersion compensation tolerance signifies the width of an allowable dispersion compensation value range for satisfaction of a certain transmission condition when compensating for chromatic dispersion by a dispersion compensator. For example, it is an allowable dispersion compensation value range for suppressing the power penalty (i.e., deterioration in the receiver sensitivity of an optical signal due to transmission) to 1 dB or less.
The chromatic dispersion of an optical transmission line varies with time due to a temperature variation, for example. The variation amount of dispersion of an optical transmission line is given by (temperature dependence of the zero dispersion wavelength of the optical transmission line)×(temperature variation)×(dispersion slope)×(transmission distance). For example, where the optical transmission line is a dispersion-shifted optical fiber, the temperature variation is −40° C. to +60° C., and the transmission distance is 600 km, 0.03 nm/° C.×100° C.×0.08 ps/nm2/km×600 km=144 ps/nm.
This value is not negligible even if an optical duo-binary modulation method with a wide dispersion compensation tolerance is employed to modulate an optical signal.
A simulation shows that with a possible transmission condition of the eye pattern penalty 1 dB or less, the dispersion compensation tolerance of an optical duo-binary modulation method of 40 Gb/s is 400 ps/nm, which is 22 km (400 ps/nm÷18.6 ps/nm/km) in terms of the length of an existing single-mode optical fiber.